1. Field of the Invention:
The present invention relates to a method of and an apparatus for printing color photographs as well as color filters for use in the apparatus. More specifically, the present invention pertains to a color photographic printing method capable of effecting printing by automatically determining, based on the printing condition of a reference film type, proper printing condition of each film type which differs in characteristic, and a color photographic printing apparatus employing such a method, as well as color filters for use in the same apparatus
2. Description of the Related Art:
Conventional types of automatic color photographic printing apparatus include a slope control function so that prints of fixed density may be produced from an overexposed or underexposed negative film The necessity of such a slope control function is derived from the reciprocity law failure of color paper, the noncoincidence of the spectral sensitivity distribution of the light measuring system of the automatic color photographic printing apparatus and that of each emulsion layer of the color paper, and the shape of the characteristic curve of each type of negative film.
The problem with the reciprocity law failure can be solved by preparing color paper with no or reduced reciprocity law failure. The problem with the spectral sensitivity distribution can be solved by making the spectral sensitivity distribution of the light measuring system to coincide with that of a corresponding exposure system. Japanese Patent Publication No. 4911/1970, Japanese Patent Laid-open No. 113627/1976, Japanese Patent Laid-open No. 64037/1978 and Japanese Patent Laid-open No. 220761/1984 disclose that the spectral sensitivity distribution of the light measuring system is made coincident with the exposure system by means of a filter or the like.
Films of some tens of types each having a different characteristic curve are presently manufactured. To cope with such various film characteristic curves, Japanese Patent Laid-open No. 29641/1974 proposes an electrical circuit for correcting the nonlinear portions of the upper and lower end portions of such a film characteristic curve to obtain an approximately linear form. This proposal, however, is not satisfactory in that no account is taken of the fact that an image is formed in the nonlinear portions. In addition, no account is taken of the way of detecting the non-linearity or the difference in the nonlinearity of each type of film.
In a case where, as described above, the spectral sensitivity distribution of the light measuring system coincides with that of the exposure system, each type of film can be properly printed based upon the printing condition of a particular film type only when the three film characteristic curves of R(red), G(green) and B(blue) are approximately parallel to the three R, G and B curves of the particular film type. In this case only, the relationship between the density of each image and a density value for exposure control which is determined from such an image density is constant irrespective of the type of film.
However, the aforesaid related art of maintaining the coincidence of the spectral sensitivity distributions involves the problem that, since the shape of each film characteristic curve is not taken into account, it is impossible to determine the optimum printing condition of a film of each type which differs in the shape of a characteristic curve.